The invention relates generally to ultrasonic transducers and more specifically concerns an ultrasonic transducer that is not sensitive to phase.
Ultrasonic measurements made on flat, parallel, and homogeneous samples are straightforward with either pulse echo or continuous wave techniques. Modern applications, however, have taken ultrasonics from the laboratory with controlled flatness and parallelism to the real world of nondestructive evaluation (NDE) and biological monitoring. Serious difficulty in interpreting ultrasonic data often arises for these modern applications. A significant cause of unusable data is phase modulation in the acoustic wave front due to inhomogeneous samples and nonparallel reflecting interfaces. For example, phase variations due to nonparallelism make accurate absorption measurements difficult if not impossible and lead to inhomogeneous broadening of mechanical resonance widths and modulation of pulse echo decay patterns.
In the past, ultrasonic measurements have usually been made with piezoelectric, magnetostrictive, capacitive, or electromagnetic ultrasonic transducers which are phase sensitive and convert acoustic pressure or strain into an electrical signal proportional to the average pressure or strain on the transducer. A phase sensitive transducer that is larger than the acoustic wavelength can lead to erroneous data since its output is modulated by phase as well as amplitude. Simply, one-half of the transducer could be detecting one acoustic wave and the other half of the transducer could be detecting another acoustic wave of different phase. For this simple case there would be an error in the output of the transducer since its output signal is proportional to average pressure.
A second class of detectors for making ultrasonic measurements include thermal converters and radiation pressure detectors. At present, these are complex, bulky, physical devices that require awkward configurations and are not appropriate for general NDE ultrasonics although they are phase insensitive.
A third class of devices for making ultrasonic measurements are the photoconductive Acoustoelectric Transducer (AET) devices which depend on photon generated charge carriers to couple to the acoustic wave. As such, they require a light source which is an important source of electrical noise due to intensity fluctuations and furthermore necessitate transparent electrodes on a CdS crystal. The conductivity in the crystal may be quite nonuniform leading to variations in the output transfer function of the crystal.
It is therefore the primary purpose of this invention to provide a simple inexpensive acoustoelectric transducer that is phase insensitive.
Another object of this invention is to provide a phase insensitive acoustoelectric transducer which does not require a light source.
Still another object of this invention is to provide both a phase sensitive as well as a phase insensitive signal for ultrasonic analysis.
An additional object of this invention is to provide a phase insensitive ultrasonic transducer that is inherently broadband.
A further object of this invention is to provide a phase insensitive acoustoelectric transducer that does not require transparent electrodes on the crystal.
Other objects and advantages of this invention will become apparent hereinafter and in the drawings.